My archive log files show period inconsistent sizes..Can someone explain whats going on here? (No - there were NO "alter system switch log files" issued during the below period. Nor were the log files altered in the below period. So that's definitely not it).

See below : the logs are sized at 500M. But noticed the archive logs on "Nov 19 18:39" or "Nov 19 21:07" or "Nov 20 00:01"..the sudden deviation in sizes..
Whats going on here? (Yes..the last one was "switched" after yesterdays backup)